Catalytic processes for the conversion of hydrocarbons are well known and extensively used. Invariably the catalysts used in these processes become deactivated for one or more reasons. Where the accumulation of coke deposits causes the deactivation, reconditioning of the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from catalyst by contact of the coke-containing catalyst at high temperature with an oxygen-containing gas to combust and to remove the coke in a regeneration process. These processes can be carried out in-situ within a fixed-bed or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a reaction zone and for coke removal in a regeneration zone are well known.
In a continuous or semi-continuous regeneration process, coke laden particles are at least periodically added and withdrawn from a bed of catalyst in which the coke is combusted. In those processes having an essentially linear progression of catalyst particles through the bed and a transverse flow of oxidizing gas coke combustion, there are regions of intense burning that extend through portions of the catalyst bed.
In a fixed-bed or batch process, coke laden particles are likewise subjected to a flow of oxidizing gas, but the gas is flowed axially through the bed instead of the particles being flowed through the gas. Similarly, there is a region of intense burning as gas travels down the bed with rising temperature as oxygen in the gas is consumed.
These regions of intense burning are important for complete coke removal, but are also difficult to control in a time efficient manner. Typically coke removal takes a significant amount of time to complete. Any extra time used to ensure complete combustion leads to production losses in an existing regenerator unit with less available reconditioned catalyst, which would otherwise be used for valuable hydrocarbon conversion reactions. Investment losses also occur for oversized equipment built at a larger size than necessary for a new regenerator unit.
For a fixed-bed regeneration process, the completion of coke burn is generally measured with a combination of bed temperature or change in oxygen concentration. Once satisfactory burn criteria have been met, a proof burn is conducted by increasing the regenerator temperature or the oxygen concentration. However, the measurement of the parameters outside the regenerator to set criteria for end of burn require scrutiny in order to avoid regenerator outlet temperature dropping below the peak burn temperature or outlet oxygen concentration increasing, which reflects dropping oxygen utilization. Dropping outlet temperatures require additional lag time to wait for increases in inlet temperature to correspondingly move down the regenerator.
Faster response times can be expected from increasing oxygen concentration, but a small increase in concentration can lead to a significant increase in peak burn temperature. Some beds have low flow areas where oxygen in the effluent can increase, while the low flow area continues to consume all of the available oxygen, thus making controlled peak burning difficult over the entire bed by merely controlling inlet and outlet oxygen concentrations.
Another problem associated with localized regions of intense coke combustion is catalyst deactivation. Exposure of high surface area catalyst to high temperatures for prolonged periods of time will create a more amorphous material having a reduced surface area which in turn lowers the activity of the catalyst until it reaches a level where it is considered deactivated. Deactivation of this type is permanent, thereby rendering the catalyst unusable. When moisture is present—water is a by-product of the coke combustion—the deactivating effects of high temperature exposure are compounded.
The combination of temperature, water vapor, and exposure time determine useful life of the catalyst. The burning of coke in localized portions of a catalyst bed has the deleterious effect of heating gases and generating moisture that pass through downstream portions of the bed and extend the high temperature exposure time of catalyst particles in the bed.
U.S. Pat. No. 3,753,926 discloses a method for regenerating a hydrocarbon conversion catalyst comprising rhenium using two carbon burning steps, where the first step is at a relatively low temperature with a small amount of oxygen and the second step is at a relatively higher temperature and a relatively higher amount of oxygen.
U.S. Pat. No. 4,507,397 discloses a sulfur removal step in a semi-continuous regeneration process prior to carbonaceous deposit oxidation. U.S. Pat. No. 4,810,683 discloses a method for regenerating a platinum containing zeolite. U.S. Pat. No. 5,155,075 discloses a low temperature method for regeneration of a platinum containing zeolite that uses a halogen-free oxygen gas.
U.S. Pat. No. 4,859,643 discloses a method for regenerating coke-contaminated catalyst particles that confines particles in the combustion section of a regenerator zone to a tapered bed configuration, which achieves better utilization of oxygen and minimizes surface area loss of the catalyst. This patent ('643) is hereby incorporated by reference into this patent application.
U.S. Pat. No. 5,001,095 discloses a method for improving a coke combustion process by segregating flue gas from the process into a high moisture content portion that is removed and a low moisture content portion that is recycled to the process.
U.S. Pat. No. 5,151,392 discloses a moving bed regeneration process with separate dispersion and chloriding steps following a coke combustion zone, which allows improved platinum re-dispersion by controlling chloride equilibrium with either oxygen-enriched or oxygen-depleted environments.
U.S. Pat. No. 5,854,162 discloses an offsite regeneration process using a moving bed furnace for a combustion step similar for used hydro-treatment catalysts, and adds a oxy-halogenation step in a sealed rotating furnace that avoids the onset of gas channeling, which improves the homogeneity of catalyst halogenation.
U.S. Pat. No. 5,965,473 discloses a method for reducing chloride emissions from a cyclic regeneration operation while saving operating costs.
U.S. Pat. No. 6,103,652 discloses a staged combustion process and apparatus for regenerating a catalyst in a moving bed that includes at least two separate successive combustion zones.